DESCRIPTION (Verbatim from the Applicant): Recent evidence indicates that extracellular calcium plays a physiological role in modulating bone remodeling through putative calcium sensing G-protein coupled receptors in osteoblasts and osteoclasts. The osteoblastic cation sensing receptor(s) may be pharmacologic targets for the induction of de novo bone formation. There is controversy, however, regarding whether the prototypic calcium receptor, CasR, or another receptor transduces these skeletal responses to calcium and other cations. We have evidence for a novel cation-sensing receptor in osteoblasts, ObCasR, that regulates osteoblast proliferative responses underlying cation-induced de novo bone formation. The existence of ObCasR is supported by the presence of a cation sensing response in osteoblasts from CasR knock-out mice, and by differences between ObCasR and CasR with regards to cell-type expression, G-protein coupling, and ligand specificity. Nevertheless, we identified a bone phenotype in CasR knock-out mice that could be attributed to either excess PTH, or lose of CasR in bone. Thus, the relative importance of CasR and ObCasR in the skeleton remains to be defined. Our overall goal is to define the skeletal function of ObCasR and CasR. We will advance the hypothesis that another cation sensing CasR-like receptor plays an important regulatory role in osteoblast-mediated bone formation by completing our characterization and cloning of ObCasR. Specific studies will: (1) investigate and compare Galpha subtype specificity and signal transduction pathways of ObCasR and CasR; (2) expression clone ObCasR by screening an osteoblast CDNA library for the extracellular cation sensing receptor(s) positively coupled to serum response element (SR-E)-mediated gene transcription; and (3) examine the function of CasR in the skeleton in CasR knock-out mice (CasR -/-) after removing the confounding effect of hyperparathyroidism by transgenic rescue of CasR deficiency in the parathyroid glands or ablation of the parathyroid glands. Defining the presence and function of cation sensing receptors in bone will permit a better understanding of physiologic and pharmacologic control of de novo bone formation by cations that may lead to new therapies for osteopenic disorders.